A prevalent characteristic of solid tumors is the presence of hypoxic areas. Intratumoral hypoxia plays a well-known role in chemo-/radio-therapy resistance and is associated with poor prognosis as well as enhanced metastasis. Hypoxia-inducible factor 1α (HIF1α) is a major mediator of the cellular response to hypoxia, which promotes malignant proliferation and progression in cancers. HIF-1α expression is increased in a variety of tumors but this is not restricted to hypoxic regions. We have previously shown that cyclin-dependent kinase 1 (CDK1) stabilizes HIF1α through direct phosphorylation of its Ser668 residue in a Von Hippel-Lindau (VHL)-independent manner both under hypoxia and at G2/M under normoxia (Warfel et al., Cell Cycle, 2013, 12, 3689-701). Another previously acknowledged VHL-independent HIF1α stabilizer is the heat shock protein 90 (HSP90) (Isaacs et al., J. Biol. Chem., 2002, 277, 29936-44) that has been correlated with adverse prognosis and recognized as a therapeutic target in cancer (Calderwood et al., Trends Biochem. Sci., 2016, 41, 311-323). We investigated herein crosstalk between CDK1-mediated and HSP90-mediated HIF1α stabilization and the involvement of therapeutic targeting.